This invention relates generally to boring tools and more particularly to self-feeding boring tools specifically designed for use in boring relatively large diameter openings in various materials such as wood for example.
Boring tools of the type generally used for machining relatively large diameter holes in wood and other similar materials commonly include a rotatable drive shaft drivingly secured to a generally cylindrical shaped member having one or more cutting members extending axially outwardly therefrom and a center pilot of self-feeding member. Typically, the cutting members include one or more spur cutters for machining the periphery of the hole and one or more generally radially inwardly extending planer blades for removing the radially inner portions of the workpiece either or both of which may be either in the form of removable cutters secured to a carrier or integrally formed therewith.
As such boring tools are generally driven by portable drill motors and in that it is normally important to insure formation of true, straight accurately located holes, it is desirable that the cutting members be circumferentially positioned so as to balance the loading on the tool around the axis of rotation so as to prevent wandering or walking of the tool during a hole forming operation. Further, as such holes may be machined in workpieces having finished surfaces on both front and back sides, it is desirable that the tool provide a clean smooth entry and exit from the workpiece without splintering or otherwise damaging the area of the workpiece surrounding the opening.
In previous designs it has been common to balance the loading on the boring tool by providing a plurality of substantially identical cutting members symmetrically arranged about the axis of rotation. However, such an arrangement of cutting members requires multiple relatively deep radially extending flutes be provided to clear chips from the work surface. The presence of these multiple flutes reduces the ability of the boring tool to resist sudden breakthrough as the cutting members approach the backside of the workpiece and thus often results in splintering, chipping, or otherwise damaging the surrounding workpiece surface area. Additionally, when sudden breakthrough occurs, substantial time and effort may be required in order to remove the boring tool from the opening, clean up the hole periphery and repair the damage to the backside of the workpiece as normally a substantial number of radially inwardly projecting splinters or fibers remain uncut within the opening after sudden breakthrough.
In other cases, the cutting members are positioned without regard to balancing the radial loading and apparently the pilot means or self-feeding means is relied upon to provide accurate tracking. However, in the event the pilot encounters an irregularity in the workpiece such as a void for example or the cutting members encounter a knot or other irregularity, an out-of-round or misaligned hole may result.
With regard to those boring tools utilizing replaceable cutting members, it has been common practice to provide different size cutting members for each diameter boring tool and to design mounting arrangements requiring removal and reinstallation of multiple fasteners for each cutting member. While this arrangement does enable continued use of the boring tool while one set of cutting members is being sharpened, it requires a substantial investment in replacement blades due to the number of different sizes required to accommodate a set of varying diameter boring tools. Further, the number and variety of spur cutter members required may create a problem into preventing loss or damage thereto when not in use.
The present invention overcomes these disadvantages in providing standard size spur cutters and standard size planer blades for use in boring tools of varying diameters, each boring tool being designed to have a pair of such spur cutters and one or more planer blades secured thereto depending on the diameter hole to be cut secured thereto. Thus, only a relatively small number of spare cutting members are required to insure continuous availability of the boring tool. Further, as each of respective types of cutting members are fully interchangeable from one diameter boring tool to another, the "spares" may be carried on different size boring tools if desired thereby reducing the possibility of the spares being lost or misplaced. The present invention also facilitates cutting member replacement and positioning procedures by providing a mounting arrangement requiring removal of only a single fastener to remove and replace each cutting member and which includes load bearing and seating surfaces which enable the replacement blade to be easily properly positioned. Further, the mounting seats are particularly designed to reduce or eliminate substantially all shear loading on the fastener thereby enabling use of a relatively inexpensive readily available and easily replaceable fastening element. Additionally, the boring tool of the present invention angularly positions each of the cutting members relative to the other cutting members in such a manner as to substantially balance the radially directed loading on the boring tool while also providing substantial surface area on the boring tool which is engageable with the workpiece to resist sudden breakthrough thereby assuring formation of a true accurate hole having relatively clean undamaged surrounding surfaces on both sides of the workpiece.
Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims taken in conjunction with the accompanying drawings.